1. Field of the Invention
This invention relates to a logic graph displaying apparatus and a logic graph displaying method, both designed for use in molding machines such as injection molding machine and die casting machine, etc.
2. Description of the Related Art
Molding machines are known, such as injection molding machine and die casting machine, each of which uses a mold. The molding machine is configured to repeat a molding cycle comprising mold closing, material injection, pressure preserving and mold opening, which are performed in a prescribed order, based on a sequential program stored in a control unit, thereby to manufacture moldings at high efficiency. To such a molding machine, various molding conditions are set, which accord with the type and material of the molding, the mold used, and the like. In the injection molding machine, for example, the mold opening, material injection, pressure preserving and mold opening are performed in sequence in accordance with the input/output (I/O) signals of ladder logic, generated by the control unit. The I/O signals therefore change from time to time, in a specific sequence.
The injection molding machine may produce, in some cases, defective moldings because of various factors. The quality of the products depends on, for example, the material (e.g., resin), mold, mold clamping force, injection speed, pressure-preserving force, molding pulling force, melding point of the material, power supply voltage, ambient temperature, wear of the machine, inadequate electrical contacts. Further, the machine may fail to operate as desired, due to a little operating-timing error resulting from the I/O signals. The injection molding machine may produce defective moldings by these factors.
If the injection molding machine produces a defective molding, the turn-on and turn-off timings of every I/O signal are checked, thereby finding out the cause of the defective molding. To check the I/O signals, it is necessary to display, on a display screen, how each I/O signal changes. Various electrical on/off signals have hitherto been used in the injection molding machine. Injection molding machines have been long known, in which how the injection molding proceeds normally or not can be observed on the display screen of the control unit.
Jpn. Pat. Appln. KOKAI Publication No. 4-133712 (Patent Document 1), for example, discloses an injection molding machine in which the on-off data of the oil-pressure pump and the control valve is displayed in the form of a logic graph. In the injection machine disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-42575 (Patent Document 2), the graphic image of an analog signal and the logic graph of an on/off signal, used in the injection molding step, are displayed. Also in the injection molding machine disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-174636 (Patent Document 3), the graphic image of an analog signal and the logic graph of an on/off signal, used in the molding step, are displayed on the same screen. Further, in the injection machine disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-199670 (Patent Document 4), a control unit is connected to a dedicated, external programmable logic controller (PLC), and records and displays the change each signal undergoes.
In the inventions of Patent Documents 1 to 3, the changes the signals undergo in one injection molding cycle are displayed on the screen. How each signal is switched on and off as scheduled can therefore be observed. How each signal changes on and off is displayed roughly, however, enabling the machine operator to grasp how one stroke of the injection molding step is proceeding. Since the screen usually has a limited number of pixels, and has a limited resolution. The screen may fail to display minute on/off change of the signal in some cases. A screen having dot size of 30 ms, for example, cannot display any signal change smaller than 30 ms.
Even if each signal changes on and off as scheduled, the injection molding machine may produce defective moldings. In this case, it would take a long time to find out the reason why. If the signal changes in an extremely short time because of the chattering due to an inadequate electrical contact or to the degradation of the valve, the change cannot be displayed on the conventional display screen.
The sequence program used may be defective and may disturb the on/off signal, but for a very short time. In this case, the signal disturbance cannot be displayed on the screen even if the on/off signal is sampled at as many intervals as possible in that very short time. From the analog waveform of a signal coming from a sensor, as displayed the screen, it is more difficult to detect such an instantaneous change of an on/off signal as specified above.
The cause of defective moldings may reside in the sequence program stored in the PLC (programmable logic controller) that is incorporated in the control unit. With the display screen disclosed in Patent Documents 1 to 3, it is difficult, in some cases, to analyze such a problem the program may have. Assume that the PLC stores a program that turns on the internal coil (contact) of a sequencer if several conditions are set in, for example, a ladder-logic sequence. The program turns a real relay contact on if the internal coil has been set to the condition specified for it. If any one of the conditions is not set for some reason, the coil will not be turned on. Then, the relay contact will not be turned on. Consequently, the relay contact seems behaving strangely even if its on/off state is stored and displayed at the control unit, disabling the operator to know the real trouble in the injection molding machine.
In the injection molding machine of Patent Document 4, the control unit is connected to an external device such as a dedicated sequencer. The behavior of the sequence program can therefore be minutely analyzed and inspected. The use of such a dedicated device, however, considerably increases the operating cost of the machine. Further, the operating efficiency of the machine at the production line is low, inevitably because the operator cannot observe the changes of each on/off signal, but at the display screen of the external device remote form the control unit of the injection molding machine.